In golf balls commercially selling, there are solid golf balls such as two-piece golf ball, three-piece golf ball and the like, and thread wound golf balls. Since the solid golf balls have excellent flight performance and durability as compared with the thread wound golf balls, the solid golf balls occupy the greater part of the golf ball market. However, in the solid golf ball, shot feel is hard and impact force at the time of hitting is large, and velocity at the time of hitting is large, which reduce the spin amount is small. Therefore, the solid golf ball is inferior to the thread wound golf ball in shot feel and controllability at approach shot.
Recently, the solid golf balls, of which flight distance can be improved while maintaining soft and good shot feel at the time of hitting as good as the conventional thread wound golf ball, generally occupy the greater part of the golf ball market. Multi-piece golf balls represented by three-piece golf ball have good shot feel while maintaining excellent flight performance, because they can vary hardness distribution, when compared with the two-piece golf ball.
Launch angle and backspin of golf ball have a great effect on trajectory of the golf ball hit by a golf club. The hit golf ball having large launch angle tends to have high trajectory, and the hit golf ball having small launch angle tends to have low trajectory. Since the backspin provides lift to the hit golf ball, the hit golf ball having large backspin amount tends to have high trajectory, and the hit golf ball having small backspin amount tends to have low trajectory. Performance requirements of golf balls from golfers include flight distance, shot feel, controllability and the like. When golfers use a golf club, particularly wood club (such as a driver), long iron club, middle iron club and the like, the flight distance is an important performance requirement.
In order to improve the flight distance when hit by a golf club such as a wood club, it is required for the hit golf ball to have high trajectory and long flight duration to a certain extent as well known. The hit golf ball having large launch angle and large backspin amount has high-trajectory as described above, but the hit golf ball having too large backspin amount tends to have short flight distance. It is reason that kinetic energy is consumed by backspin, and that force applied such that the hit golf ball is pulled backward occurs by the lift until the golf ball reaches the highest point of the trajectory because the lift is applied perpendicular to the flight direction of the golf ball. Therefore, golf ball, of which the backspin amount is not very large and high trajectory is accomplished by high launch angle, has long flight distance when hit by a golf club, such as a wood club.
Based on the above knowledge, there has been many developments of golf ball having long flight distance accomplished by low backspin amount and high launch angle at the time of hitting, while maintaining the other properties, such as good shot feel, controllability and durability, from the viewpoint of formulation of the material and structure of the golf ball (Japanese Patent Kokai Publication Nos. 38238/1997, 239068/1997 and the like).
In Japanese Patent Kokai Publication No. 38238/1997, a golf ball comprising a core and a cover covering the core is disclosed. The cover has a two-layer structure composed of an outer cover and an inner cover, the inner cover is prepared from a resin composition having a flexural modulus of 5,000 to 12,000 kgf/cm2 at 23° C. and a relative humidity of 50%, and comprising a polyamide resin having a flexural modulus of 6,000 to 30,000 kgf/cm2 at 23° C. and a relative humidity of 50% and a thermoplastic elastomer having a JIS-A hardness of 30 to 98, in a weight ratio of polyamide resin : thermoplastic elastomer within the range of 95:5 to 50:50.
In Japanese Patent Kokai Publication No. 239068/1997, a three-piece solid golf ball comprising a solid core, an intermediate layer and a cover is disclosed. The core has a center hardness in JIS-C hardness of not more than 75 and a surface hardness in JIS-C hardness of not more than 85, the surface hardness is higher than the center hardness by 8 to 20, and in the layer hardness in the JIS-C hardness is higher than the surface hardness of the core by not less than 5, a cover hardness in JIS-C hardness is lower than the intermediate layer hardness by not less than 5, and a ratio of the golf ball surface area occupied by the dimple to the total surface area of the golf ball is not less than 62%.
However, it has been required to improve the performances of the golf balls still more by golfers. Therefore, there has been no golf ball having excellent flight performance by accomplishing small backspin amount and high launch angle at-the time of hitting, while maintaining the above other properties.